ata_piix: constify pci_bits

pci_bits are not supposed to change at runtime. Functions
pci_test_config_bits() working with const 'struct pci_bits'.
So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>